<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Demo_flex</title>
    <style>
        .box1{
            width: 400px;
            height: 400px;
            display: flex;
            justify-content: center;
            background-color: blue;
            text-align: center;
        }
        .box2{
            width: 100px;
            height: 100px;
            background-color:aquamarine;
        }

        </style>

</head>
<body>
    <div class="box1">

     <div class="box2">helloworld</div>
     <div class="box2">helloworld</div>
     <div class="box2">helloworld</div>
    </div>  
</body>
</html>